Field of the Invention
The present invention relates to mobile communication.
Related Art
3rd generation partnership project (3GPP) long term evolution (LTE) evolved from a universal mobile telecommunications system (UMTS) is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) evolved from the 3GPP LTE.
As disclosed in 3GPP TS 36.211 V10.4.0 (2011-12) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10)”, 3GPP LTE/LTE-A may divide the physical channel into a downlink channel, i.e., a physical downlink shared channel (PDSCH) and a physical downlink control channel (PDCCH), and an uplink channel, i.e., a physical uplink shared channel (PUSCH) and a physical uplink control channel (PUCCH).
Meanwhile, as more communication devices require more communication capacities, efficient utilization of limited frequency band in the next-generation wireless communication system has become more important requirement.
In a cellular communication system such as the LTE system, it is also considering a method to utilize, to offload traffic, an unlicensed band such as 2.4 GHz band on which an existing WLAN system uses or an unlicensed band such as 5 GHz band which has recently got attention.
Basically, since the unlicensed band assumes a method of transmitting and receiving wirelessly with a contention between each communication node, it requires to confirm that other communication nodes do not transmit a signal, by performing, by the each communication node, a channel sensing, before transmitting a signal. This is called as a CCA (Clear Channel Assessment), and a base station (eNB) or an user equipment (UE) of the LTE system should be also able to perform the CCA to transmit a signal in the unlicensed band (for convenience, referred to as a LTE-U band). Thus, that base station or UE may be referred to as an LTE-U base station or UE.
Further, the other communication nodes such as WLAN etc., as well do not trigger any interference, by performing the CCA, when the eNB or the UE in the LTE system transmit a signal. For instance, a CCA threshold in the WLAN standard (801.11ac), is regulated to be as −62 dBm for the non-WLAN signal, and −82 dBm for the WLAN signal, which means that the STA (Station) or the AP (Access Point), for instance, does not transmit a signal in order not to trigger any interference, if any signal other than the WLAN signal is received with the power of more than −62 dBm.
Specifically, the STA or the AP, in the WLAN system, may perform the CCA and a signal transmission unless any signal with the level of greater than CCA threshold, is detected for more than 4 us.
Specifically, a process is started when a node wishing to transmit data transmits RTS (Request To Send) frame in WLAN.
A destination node responds to the signal by transmitting a CTS (Clear To Send) frame, in a wireless environment when there are no other signals of being transmitting and receiving and thus the transmission is available.
All other nodes receiving the RTS frame or the CTS frame is prevented from transmitting data for a determined time. A time when the transmission is prevented may be indicated by information included in the RTS frame or the CTS frame. This protocol set forth a premise that all nodes have the same transmission range.
RTS/CTS are an additional and selective method for implementing CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) virtual carrier sensing.
Since only a physical carrier sensing is used in the basic 802.11 (WLAN) standard, there is a problem that a hidden node or terminal problem may be occurred.